In view of the advantages of capacity improvement and coverage of blind-spots brought by low-power nodes, there is a big interest in deployment and enhancement of small cells in LTE/LTE-A (LTE-Advanced). In 3GPP R12, a new research project, “Small Cell Enhancements for E-UTRA and E-UTRAN-higher-layer aspects”, has been approved and the important significance of it lies in supporting multiple link connections to macro cells and small cells. If there are multiple link connections established for one user equipment (UE), such as a first connection established to a macro cell and a second connection established to a pico cell which is a typical small cell, then this UE can avoid frequent switching among pico cells which are deployed in high density, via the first connection to the macro cell, and meanwhile obtain a throughout gain via the second connection to the pico cell.
As well known, a base station has a crucial function for scheduling to allocate resources to users so as to achieve desired QoS and system capacity. Power Headroom Report (PHR) is one important input for uplink scheduling. The importance of the power headroom report in an uplink system is to provide a base station (e.g. eNB) with some important information required for scheduling uplink resources, that is power information on the UE side. In the existing systems, generally there is only one serving base station for one user, therefore the user only needs to send to one base station the power headroom report of every active cell. However, if two or more link connections need to be supported, the user needs two or more serving base stations and two or more independent schedulers (in the case that two or more base stations are linked through a non-ideal backhaul), thus the user needs to report power headroom to two or more base stations.
In the existing systems, since there is only one serving node or base station (although with carrier aggregation (CA), there are multiple serving cells, these cells are co-located physically), power headroom reporting is relatively simple for the following reasons, e.g.:
only one serving node or base station needs to obtain the power headroom to schedule resources;
since only one group of triggering conditions need to be maintained for one link, even in the case with CA, only one group of triggering conditions need to be maintained during the power headroom reporting; and
since the physical uplink link control channel is transmitted only on the primary carrier or primary cell (PCell), only one power headroom report of type 2 is needed.
However in the case of enhanced small cells, i.e. in the case of requiring support of multiple link connections, some new problems on the power headroom reporting arise and need to be solved, e.g.:
Due to the presence of two or more serving base stations (for example, a macro cell base station and a small cell base station), a user needs to report power headroom to these two or more base stations, respectively. In a single carrier system, the user needs to obtain two pathloss values respectively for the carriers configured for the macro cell base station and the small cell base station and to perform a power control so as to obtain transmitting power, and to calculate the power headroom respectively for the two carriers of the two base stations. In the case of multi-carrier aggregation, each base station is configured with multiple carriers. The user needs to obtain different transmitting power on different carriers of different base stations and calculate based on them the power headroom for each carrier of each base station. Thus, when the user needs to send power headroom reports to more than one base station simultaneously, it is necessary to solve such problems as what value is to be reported to each base station and how to report it.
Therefore, the existing power headroom reporting mechanism cannot support two or more link connections. There is a need for a novel power headroom reporting mechanism taking into consideration new characteristics of dual/multiple link connections